Information is frequently transmitted by systems which modulate the phase or frequency of a carrier signal. This type of modulation is used both for signals transmitted over wires and for radio signals transmitted via electromagnetic radiation. A receiver of such signals frequently must perform a filtering operation on the transmitted signals to eliminate noise and signals not within the frequency band of the transmitted signal. When digital data is transmitted by means of well-known phase-shift keying (PSK) or frequency-shift keying (FSK) techniques, in addition to a filtering operation, the receiver system must also provide a conversion of the phase information to digital form.
Other applications in which the information is encoded on the phase of a signal include navigation systems, such as LORAN and OMEGA systems, and systems using angle readout transducers, such as resolvers and synchros (or selsyns) where the mechanical angle of a rotating shaft is converted to an electrical phase angle by the operation of the transducer. In such systems it is important that the phase angle of the signal be recovered very accurately. Also, in such applications there may be requirements for filtering and/or conversion of phase information to digital form.
Frequently, phase-decoding systems require very narrow bandwidths relative to the modulated frequency, and such narrow bandwidths are often difficult and expensive to obtain. Moreover, in applications where filtering is required, it would be advantageous if the bandwidth can be varied dynamically without detrimentally affecting the performance of the phase detection circuitry. Thus, in a navigation system such as LORAN or OMEGA, the received signal may have a varying signal-to-noise ratio (S/N) and in such cases it is desirable to have a relatively wide bandwidth so as to decrease the response time of the system when S/N is high and to have a relatively low bandwidth in order to achieve the desired accuracy when S/N is low. Further, in applications where it is important to accurately recover the phase of a signal as quickly as possible after the signal has been detected, phase errors due to the transient response of the phase-demodulation circuitry can be greatly reduced by dynamically changing the bandwidth in an optimal manner.
A system which can achieve the requirements mentioned above and which can be relatively easily adapted for different applications would prove to be a useful device. The value of such a system would be further enhanced if it could be relatively easily and inexpensively implemented in integrated circuit form.